Dissonance of the Nexus DLC
This is missable.
All credit to Omega N1ghtm4r3 for determining the exact details of the dialogue choices
1.) When you 1st encounter LLenn, Fukaziroh, Pitohui, and M outside of the shop in the SBC Glocken, Rain and Seven soon joins the conversation. You'll finally get the rewards from the main quest, Battle Preparations.
Immediately afterwards, Seven and Rain will bring up something about gathering some materials, you have to select the 2nd option that says, I know where to look. This will give you the quest, Outerwear Materials: Hard
2.) After using the Night Crystal to save Sachi, you'll end up completing the Frozen Door main quest. Afterwards, Eiji will start to walk away due to him looking for someone. When Kirito offers help to search, select the 2nd option, We'll search every last inch of this place. This will net you the main quest, Manhunt: Hard.
3.) After doing both of the above, you get a new mission after the Lievre Material Gathering Request that you otherwise wouldn't receive. The new quest is called, Yuna's Request. Doing this mission unlocks the Chic Dress F Design. Afterwards you will also go through the cut-scene for the e-mail for the Until Next We Meet achievement.
(Edit: will be attempting to get TA to mark these achievements as missable.)